Gas fees: what they are, why they change and tax

A gas fee is what you pay for the network to process a transaction. Beginners find this out the hard way: they hold USDT in a wallet, try to send it and the transaction will not go through because there is no ETH to pay the gas. Or they send R$ 50 and pay R$ 15 in fees because they picked the wrong network at the wrong time. This guide explains what the fee is, how it is calculated on Ethereum and Bitcoin, why it swings so much, how to pay less and what to do with it at tax time.

What gas measures

On Ethereum, gas is the unit that measures the computational effort of an operation, according to the network's own documentation (ethereum.org, checked in October 2026). A simple ETH transfer costs 21,000 units of gas; interacting with a contract, such as sending a token or using a decentralized exchange, costs more, because it requires more operations. The amount paid is the number of units multiplied by the price per unit, measured in gwei, which is one billionth of an ETH.

Since the EIP-1559 upgrade in 2021, the price has two parts. The base fee is set by the protocol and moves up or down by at most 12.5% per block depending on whether the previous block was fuller or emptier than the target; it is burned, meaning it leaves circulation. The priority fee is the tip you offer validators to get in sooner. Total fee = gas units × (base fee + priority fee).

On Bitcoin the logic is similar under a different name: the fee is measured in satoshis per virtual byte (sat/vB) and depends on the transaction's size in bytes, not on the amount sent. Sending 10 bitcoin or 0.001 bitcoin costs the same if the transaction has the same size. What changes is the queue: when the mempool, the waiting room for transactions, is full, whoever pays more goes first.

Why the fee changes so much

Three factors explain almost all of the variation.

Demand for the network. In moments of euphoria, a token launch or a sharp price drop, everyone wants to transact at once, and the base fee climbs block after block. On weekends and overnight, it falls.

Complexity of the operation. Sending ETH costs 21,000 units; sending an ERC-20 token costs around 65,000; a swap on a decentralized exchange can pass 150,000. The same network, in the same minute, charges different amounts depending on what you do.

The network you choose. This is the biggest difference. Ethereum's documentation cites an average cost of US$ 0.12 per transaction on mainnet against US$ 0.0016 on layer 2 networks. That gap exists since the Dencun upgrade of March 13, 2024, which created "blobs", a cheap data space that layer 2s use to publish their transactions: fees on Arbitrum, Optimism and Base fell more than 90% in the week of the upgrade. What those networks are and what they change for self-custody is in layer 2: what it is and what changes for self-custody.

How to pay less without mistakes

Four practical rules.

1. Hold the network's native coin. USDT on Ethereum pays gas in ETH; on Tron, in TRX; on Solana, in SOL. A wallet with the token but no native coin is stuck. Always keep a small buffer.
2. Pick the network by fee, but check the destination. Withdrawing over the cheapest network only works if the destination wallet or exchange accepts that network. Network mistakes are the subject of sent crypto on the wrong network: can it be recovered?.
3. Look at the fee before confirming. Wallets show the estimate. If the cost looks out of proportion to the amount, wait a few hours or switch networks.
4. Do not use the "minimum" Bitcoin fee when in a hurry. A transaction with too low a fee can sit in the mempool for days. For amounts with no rush, a low fee is great; for urgent ones, it is not.

At Vault's Self-Custody Immersion, the sending and receiving practice covers exactly this: choosing a network, estimating the fee, sending a test amount and confirming arrival, with someone watching.

How gas fees enter Brazilian taxes

There are two situations here, and the difference matters.

A fee paid on a purchase or sale. The network fee and the exchange fee are part of the cost of the operation. On a purchase, they add to the acquisition cost; on a sale, they reduce the amount received. That is the usual treatment of transaction costs in capital gains, and it is how specialized accountants compute it.

A fee paid on a transfer between your own wallets. Moving from an exchange to your wallet is not a sale, so it creates no gain. But the fee was paid in crypto, and technically that fraction of ETH or bitcoin is no longer yours. The prevailing reading is to treat the fee payment as a tiny disposal of that asset, which in practice almost never generates tax because the amount is small and fits within the monthly exemption of R$ 35,000 in sales. What matters is that it shows up in the history, so the declared position matches the wallet.

Rule checked in October 2026, based on Normative Instruction RFB 1,888/2019; there is no specific guidance from the tax authority on network fees, so the conservative approach is to record everything. Vault Tax reads wallet transactions and handles fees automatically; the guide crypto taxes in Brazil explained covers the rest of the computation.

Crypto assets carry high risk, including significant volatility and the possibility of losing the invested capital. Vault Capital is a securities advisory firm authorized by Brazil's CVM under CVM Resolution 19/2021 and never holds custody of client assets.

Frequently asked questions

Why can't I send my USDT?

Almost always because there is no native coin to pay the fee: ETH on Ethereum, TRX on Tron, SOL on Solana, BNB on BNB Chain. Buy a small amount of that coin, send it to the wallet and the USDT transfer works.

Does the gas fee depend on the amount sent?

No. On Ethereum it depends on the complexity of the operation and demand for the network; on Bitcoin, on the transaction's size in bytes and the queue. Sending R$ 100 or R$ 100,000 costs the same fee under the same conditions.

Which network has the lowest fees?

Ethereum's layer 2s (Arbitrum, Optimism, Base) and networks such as Tron and Solana charge cents or fractions of a cent. Ethereum mainnet and Bitcoin charge more, especially at peak times. A low fee only helps if the destination accepts the network.

Are gas fees deductible for Brazilian income tax?

A fee paid on a purchase or sale enters the cost of the operation and reduces the capital gain. The fee on transfers between your own wallets is a small disposal of the asset used to pay it; record it in the history, even if it generates no tax.

Let's talk

If you want to learn to choose a network, estimate the fee and send without fear, Vault's Self-Custody Immersion does it hands-on, in one weekend, in person, with the setup validated by a specialist.